1. Field of the Invention
The present invention relates to an improvement in chemical cutting systems and more particularly to a new and unobvious pressure bleed-off system for an apparatus which chemically cuts objects down within a well bore to insure that the center does not get stuck down in the hole.
2. Prior Art & General Background
The most relevant prior art of which Applicant is aware may be found in U.S. Pat. No. 4,250,960 (Chammas, issued Feb. 13, 1981) and No. 4,620,591 (Terrell et al, issued Nov. 4, 1986); both of these patents teach a down hole chemical cutting system having pressure relief means distinguishable from the present invention and will be further discussed infra.
A list of prior patents which may be of interest are listed below:
______________________________________ U.S. Pat. No. Patentee(s) Issue Date ______________________________________ 2,918,125 Sweetman, W. 12/22/1959 3,076,507 Sweetman, W. 02/05/1963 3,211,093 McCullough 10/12/1965 4,125,161 Chammas 11/14/1978 4,158,389 Chammas et al. 06/19/1979 4,180,131 Chammas 12/25/1979 4,250,960 Chammas 02/17/1981 4,315,797 Peppers 02/16/1982 4,415,029 Pratt et al. 11/15/1983 4,494,601 Pratt et al. 06/22/1985 4,619,318 Terrell et al. 10/28/1986 4,620,591 Terrell et al. 11/04/1986 ______________________________________
As may be ascertained by the above listing of prior art, chemical cutters are well known, having been in use at least as early at 1959, when W. G. Sweetman invented one of the first practical downhole chemical cutting apparatus and methods.
These cutter devices are frequently used to cut, sever, perforate or slot an object down within a well bore. The cut object referred to above may include drill pipe, casing, or foreign objects which may become lodged in the well bore.
The above referenced cutter devices generally comprise tubular casings which contain a highly corrosive or oxidizing chemical cutting fluid. When the fluid is introduced to the desired discharge area, it reacts violently with it, totally oxidizing that portion of the area which the chemical contacts.
Thus, when the cutting fluid is properly introduced to the well bore, the chemical should effectively sever the drill pipe and/or casing. The most widely used chemical cutting fluids have been fluorine or halogen fluorides, including but not limited to chlorine monofluoride, chlorine trifluoride, bromine pentafluoride and other compounds.
The chemical cutting fluid is generally contained in a cylindrical containment/discharge vessel which is lowered within the bore to the desired discharge area. The fluid is then applied by utilizing a pressurizing agent, typically black gunpowder or the like, which causes a high pressure discharge.
The measure of the pressurizing agent, taking into consideration the amount of cutting fluid, the size of the discharge ports, and the hydrostatic pressure at the cutting depth, may be calculated in such a manner as to provide an accurate and effectively controlled means of introducing the cutting fluid to the desired area.
In addition to the above teachings, much of the prior art has also taught a means of securing the chemical cutting apparatus to the well bore during the discharge phase, locking the apparatus in place, thereby providing a more accurate application of the cutting fluid to the desired area.
The method has generally comprised the use of anchors or slips which are forced outward from the periphery of the cutting apparatus and against the well bore in response to the increased pressure generated by the pressurizing agent during discharge.
The internal pressure generated during the discharge phase is released through the chemical discharge ports, once the cutting fluid has been expelled.
After the discharge phase is complete and the excess internal pressure is dissipated, springs and/or the hydrostatic pressure of the well and/or other means cause the anchors or slips to retract to their closed position, disengaging the cutting apparatus from the well bore.
A frequent problem which has occurred over the last twenty to thirty years in this art arises when the chemical discharge ports become clogged before or during the cutting operation. Such an occurrence not only hinders or eliminates the proper discharge of the cutting fluid, it also prevents the internal pressure generated by the ignition of the pressurizing agent from being dissipated through the discharge ports, thus causing the anchors or slips to remain frozen in their fully erect or locking position.
A practical consequence of this malfunction is that the anchors or slips remain in an anchoring position against the well bore, making removal of the cutting apparatus difficult if not impossible. Another consequence is that the cutting apparatus becomes very dangerous to handle, and, even if the apparatus is able to be removed, it must somehow be manually vented, which might be impossible to safely accomplish, depending upon its design.
Thus, a longstanding need arose for a chemical cutting tool which included a means for venting or bleeding off excess pressure which remained due to a malfunction caused by a clogged or otherwise blocked discharge port.
As briefly cited supra, U.S. Pat. No. 4,620,591, issued Nov. 4, 1986 and No. 4,250,960, issued Feb. 17, 1981 attempted to fulfill the long felt need for such a pressure relief system. These, it is believed, are the only known prior teachings in this art which teach pressure venting or bleed-off means.
U.S. Pat. No. 4,250,960, entitled "Chemical Cutting Apparatus" teaches an improvement residing in a pressure relief subassembly situated between the pressure generation assembly and the chemical discharge assembly which comprises a valve mechanism for selective restriction of an aperture in the sub body, said mechanism allowing manual venting of the apparatus.
The operation of the system is more fully explained in the following quote, found under column 10 of the U.S. Pat. No. 4,250,960:
"Upon retrieval of the tool from the well, and if the tool failed to operate for any reason such as the firing sub not functioning, pressure relief sub is vented first by opening the valve means by unscrewing the shem. Then, if the tool should fire accidentally during handling, substantial pressure would be vented through the aperture and out the opening. Thus, the pressure relief sub functions to greatly reduce the risk of injury to personnel."
Thus, the above pressure relief means requires total manual venting of the apparatus, which is not only exceedingly dangerous but may also prove impossible, if the device is anchored to the well bore. Therefore, said "improvement" does not attempt to fulfill the need for a safer and more effective downhole pressure relief means, which would allow the anchors or slips to disengage from the erect position, unfreezing the apparatus from the well bore.
U.S. Pat. No. 4,620,591 issued 1986 and entitled "Chemical Cutting Apparatus Having Selective Pressure Bleed-Off" attempts to fulfill the need for downhole pressure relief via a rather mechanically complicated, selective bleed-off subassembly.
U.S. Pat. No. 4,620,591 teaches a "selective" means of downhole venting, presumably to be used only in those cases where the apparatus actually becomes lodged downhole due to the clogging of the discharge port(s).
The patent utilizes two sub-assemblies comprising one sub and held together by two shear screws and locked in place by two steel balls which are configured in a retaining position prior to firing. Once the propellant or charge is ignited, the retaining balls are forced by the increased pressure into a position which allows the sub-assemblies to be slidably separated in the event of a misfire.
In the event of a misfire, the anchor slips would become lodged against the pipe wall, making removal of the device difficult, if not impossible. However, with the above patent, the relocation of the steel balls due to the firing of the propellant charge would allow the operator to selectively bleed-off the excessive pressure in the subs by pulling the wireline in an attempts to retrieve the device.
Sufficient upward pressure would act to shear two shear screws and cause the two sub-assemblies to telescope apart (as the retaining steel bolts have changed position due to the discharge of the propellant), allowing a bleed-off aperture to be exposed, thus venting the excessive pressure from the subs. At that point, the anchor slips would be forced back into their retracted position by the well's hydrostatic pressure. The apparatus would then be free to be pulled from the hole, with the excessive pressure drained and thus less hazardous to diffuse.
However, this system not only requires reassembly and changing of the shear bolts once it has been selectively vented, it also is mechanically rather complicated and thus should require more maintenance and have a substantially greater risk of failure than the relatively "fail safe" present invention discussed infra.
3. General Summary Discussion of the Invention
The present invention overcomes these prior art problems by providing a system which is highly reliable, relatively economical and very cost effective.
Although chemical cutting systems have been in use now for over thirty years, one of the first being invented by W. G. Sweetman sometime before 1955, the industry has, until now, been unsuccessful in perfecting a downhole pressure bleed-off system which is reliable, automatic, relatively maintenance free and cost effective.
Further, the present invention should provide a significant, substantial commercial impact with regard to the design and implementation of downhole chemical cutters due to the effectiveness of this system.
U.S. Pat. No. 4,620,591 appears to be the only prior art which teaches a means of downhole pressure relief in the event of a malfunction but, as discussed supra, it teaches a rather expensive, complicated, and potentially unreliable means when compared to the present invention. The present invention fulfills a long felt need which the industry has strived for since the invention of the chemical cutter, as will be shown infra.
The present invention comprises in its preferred embodiment the implementation of a pressure relief port in the anchor or slip sub-assembly, located between the propellant and chemical cutting fluid vessel assemblies, said relief port being for example one eighth (1/8) of an inch in diameter as used with for example the standard size 1 11/16 chemical cutter assembly (but varying in size depending upon the size of the assembly) in conjunction with an expendable plug, sealant, and "O" ring seal configuration.
When the cutter is fired the pressure from within the sub pushes the anchor slips out, said pressure then communicating with the pressure relief port. The pressure then pushes the expendable plug out of the relief plug chamber, thereby allowing venting to begin. Even though venting has begun, the size of the relief port is such that the internal pressure remains high enough to satisfactorily convey the cutting fluid through the chemical discharge port(s).
If the chemical discharge ports become clogged to such a degree as to prevent discharge of the cutter, the pressure would ultimately vent through the relief port, thereby allowing the hydrostatic pressure of the well to push the anchor slips into the retracted position, thus "freeing" the apparatus.
Without this venting effect, the cutter could either explode due to the tremendous amount of internal pressure generated during the firing of the propellant or the anchor slips would remain frozen in a fully erect position, preventing removal of the apparatus from the well bore. The present invention solves the problems associated with the above scenario in the most uncomplicated, reliable, and cost effective means to date.
The pressure generated by the firing of the propellant in the exemplary embodiment is approximately 3500 PSI. As only 1500 PSI is required to rupture the containment membrane of the chemical sub-assembly, there is more than enough pressure to complete the cutting operation even with the utilization of the relief port.
Further, the relief ports positioning in the anchor slip chamber is such that the pressure does not communicate with the relief port until the anchor slip has erected into its locking position. This is because the erection of the anchor slip occurs simultaneously with the application of the chemical cutting agent.
Thus, the loss of pressure from the use of the relief port is negligable and does not affect the performance of the cutter. Numerous experamental test cuttings in the field verify that the implimentation of the design as taught in the exemplary embodiment of the present invention works consistantly and in a much superior manner than the prior teachings.
Further, the numerous field test cuttings also verify that this is by far the most efficient, least troublesome, and most reliable cutting system to date, with none of the failures associated with the prior art.
It is thus an object of the present invention to provide an improved chemical cutting apparatus for utilization within a well bore.
It is another object of the present invention to provide an improved cutting apparatus for utilization within a well bore, which permits pressure within the tool to be automatically vented.
It is another object of the present invention to provide a safe, reliable, and cost effective means for automatic pressure venting without affecting the cutting effectiveness of the cutters.
It is yet another object of the present invention to provide a safe, reliable, and cost effective means for automatic pressure venting which could be adopted for use with many of the present cutting systems on the market.